Studies will be continued on the allosteric regulation of dihydroorotase from a variety of sources. Special attention will be focused on the constitutive enzyme from C. oroticum. In addition, work will continue on the role of divalent cations in dihydroorotase and dihydrouracil amidohydrolase catalysis. We are especially interested in determining if the enzymes go via "Zn-carbonyl" or "Zn-hydroxide" mechanisms. The metabolism of the halogenated dihydrouracils will be studied.